Booster seat law by height and weight

A booster is a child restraint, not a step on the way to an adult belt. GOV.UK includes high-backed booster seats and booster cushions in R44 groups 2 and 3, and R129 height-based boosters on their own labels. The child uses one until they reach 135 cm or their 12th birthday, whichever happens first, unless a statutory exception applies.

Highway Code rule 100 says that if a child is under 1.35 metres, a baby seat, child seat, booster seat or booster cushion must be used, suitable for the child's weight and fitted to the manufacturer's instructions. Age charts that say 'from 4 years' are marketing, not the legal test.

Moving a child into a booster the day they can sit without a harness is a common mistake. They still have to meet the booster's labelled height or weight. An R44 group 2 seat starts at 15 kg. An R129 booster starts at whatever height its label prints, often around 100 cm, but always read the seat.

Group 2 is 15 kg to 25 kg. Group 3 is 22 kg to 36 kg. Both groups list a rear or forward facing child car seat, high-backed booster or booster cushion, using a seat belt, harness or safety shield. The ranges overlap, so a child at 23 kg sits in both groups. The seat's own approval range still decides whether that particular booster is legal.

GOV.UK also notes that manufacturers can now only make booster cushions approved as group 3. That does not withdraw existing group 2 cushions. It changes what new cushion models can be introduced.

A high-backed booster guides the diagonal belt across the shoulder and keeps the child's head in line with a side wing. A backless cushion only lifts the child. Both are child restraints when they are approved and labelled for this child. The backless rules from 9 February 2017 are on the backless booster page.

Most boosters use the vehicle's diagonal seat belt to hold the child. GOV.UK says you must only use a child car seat if that belt has a diagonal strap, unless the seat is designed for a lap belt or uses ISOFIX. A lap belt on its own, on a booster that needs a diagonal strap, is not a legal fit.

The belt must sit on the shoulder, not the neck, and across the hips, not the stomach. High-backed boosters have belt guides for that purpose. If the diagonal section cuts the neck even with the guide at its highest setting, the child has not yet reached a geometry this booster can provide, or this vehicle belt is the wrong length. Do not put the belt under the arm or behind the back.

From their 12th birthday, or from 135 cm, whichever comes first. Until then a booster or other approved child restraint is required in the family car, with the usual exceptions for taxis, unexpected short journeys for children aged 3 or older, and no room for a third seat.

A child who is 11 and 136 cm may use the adult belt. A child who is 12 and 128 cm may also use the adult belt. A child who is 11 and 128 cm still needs the correct child restraint.

Once the adult belt is the legal restraint, it must be worn where one is fitted. The driver remains responsible until the child is 14.

The driver of a company car or hire car has the same duty. If the hire car has no ISOFIX, a belt-fitted booster still works if the rear belts have a diagonal strap. If the rear has only lap belts, that booster cannot go there unless it is designed for a lap belt.

If there is no room for a third child car seat in the back, children aged 3 or older can sit in the back using an adult belt. That exception is about space for a third restraint, not about a child who simply prefers the belt. Children under 3 must still be in a child car seat, which may mean using the front with the correct seat.

The diagonal belt is tucked under the arm because it 'feels tight'. That is not wearing the belt as the restraint requires.

A backless cushion is used for a child under the 125 cm and 22 kg thresholds on a new model, or for a child whose older group 2 cushion label does not cover them.

The booster is placed in front of an active airbag with a child who still needs a rear facing seat. Boosters are not rear facing baby seats, but a live airbag is still a vehicle-handbook problem for a small child in the front.

The child is moved to the adult belt at 135 cm measured in shoes on a carpet. Measure height without shoes, and keep the booster until you are sure they have reached 135 cm or 12.

Myth: high-backed boosters are only a comfort extra. In R44 groups 2 and 3 they are a listed child seat type.

Myth: once they can sit in a shopping trolley they can use a cushion. Weight and height on the label decide, not shopping-trolley posture.

Myth: the MOT will fail a car with the wrong booster. An MOT does not inspect child restraints.